The art shows a development in the field of cleaning the exhaust air from paint and lacquer spray booths ranging from the description in U.S. Pat. No. 3,421,293, to more sophisticated and complex arrangements as will be described below.
In the approach of U.S. Pat. No. 3,421,293, the apparatus for the removal of paint from the exhaust air of a paint-spray booth provides below the grate-like floor of the spray booth means for bringing the exhaust air passing downwardly through into intensive contact with wash water which is recirculated. After separation of the exhaust air from the water, the paint is carried off in the wash water. The wash water is treated with chemicals to improve the washing effect and to permit the wash water to be cleaned, i.e. to allow the paint particles to be removed from the wash water.
German patent document DE-AS 21 61 198, DE-AS 24 12 988 and DE-PS 28 14 276 describe respective particles which operate under the same principle without describing in each case the wash water recirculation. In each of these documents, emphasis is placed on the shape of the apparatus which is intended to ensure intimate contact between the exhaust air and the wash water. Utilizing the techniques described in these documents, improvements in air cleaning efficiency at reduced noise levels can be obtained.
European patent 0 047 432 describes an exhaust-air scrubber as the first cleaning stage for the exhaust air and provides thorough scrubbing in this first stage. It mentions that a second cleaning stage can be provided following the first and which includes a wet electrostatic filter or precipitator supplied with a rinse liquid, the cleaned air being partially recirculated to the spray booth.
German patent DE-PS 30 42 464 describes a technique based upon the principle of reducing the energy consumption of a conventional scrubber or the principle of improving the air-cleaning efficiency for a given energy consumption. This also requires a particular shape of the scrubber housing and utilizes a constriction of special form to permit intimate contact of the wash water with the exhaust air.
Finally, mention may be made of German patent document DE-OS 35 17 392 which proposes, after the initial intensive scrubbing, a second scrubbing stage which has a constricted portion in which a water jet is atomized through a Venturi tube at the region of the smallest cross section. Downstream therefrom a separator is provided at which the water is recovered for recycling. Alternatively, a further cleaning stage is provided whereby a portion of the cleaned exhaust air is recycled to the lacquer-spray booth.
German patent DE-PS 30 42 464 describes a technique based upon the principle of reducing the energy consumption of a conventional scrubber or the principle of improving the air-cleaning efficiency for a given energy consumption. This also requires a particular shape of the scrubber housing and utilizes a constriction of special form to permit intimate contact of the wash water with the exhaust air.
Finally, mention may be made of German patent document DE-OS 35 17 392 which proposes, after the initial intensive scrubbing, a second scrubbing stage which has a constricted portion in which a water jet is atomized through a Venturi tube at the region of the smallest cross section. Downstream therefrom a separator is provided at which the water is recovered for recycling. Alternatively, a further cleaning stage is provided whereby a portion of the cleaned exhaust air is recycled to the lacquer-spray booth.
Invariably these earlier processes utilize, therefore, as the first cleaning stage, an intensive scrubbing stage in which the shape of the scrubber determines the intimacy of contact of the scrubbing liquid with the exhaust air. In some cases, such a conventional high-intensity and high-velocity scrubber is the sole air cleaning unit, while in other cases, further air-cleaning systems are provided or required. See, for example, the above-mentioned use of the wet electrostatic filter spatially separated from the scrubbing stage and rinsed with a separate flow of water.
All of these systems have disadvantages. For example, in the high-velocity scrubbers constituting the usual first or exclusive cleaning stage, a high pressure drop or loss is unavoidable because of the need for a constriction in the scrubber to ensure intense interaction of the sprayed water and the exhaust air. In addition, or as a correlative to the high-velocity and high-pressure drop, which are associated with high energy consumption, there is a high noise level. Furthermore, where the scrubber with its constriction is provided immediately below the spray booth, the presence of a constricted flow of the exhaust air downstream of the spray booth has a detrimental effect on the manner in which the air is supplied to or passes through the spray booth.
There are also disadvantages on the water side of such apparatus. For example, relatively large amounts of water must be used to ensure thorough flushing of the large funnel-shaped constriction-forming surfaces of the scrubber portion of the apparatus to thoroughly flush the surfaces. As a consequence correspondingly large units, such as settling basins or apparatus for the active separation of the lacquer particles from the wash water, must be provided. In addition, because large volumes of water must be used, correspondingly large quantities of the chemicals which may be required to treat the wash water must be employed as well. These chemicals include surface-active, flocculating, emulsion-breaking and like agents ensuring agglomeration of the paint and lacquer particles and easier separation thereof from the wash water so that wash-water cleaning is more efficient and can be effected to a higher degree.
The utilization, for example, of chemical flocculating and settling agents can permit more economical recovery of the separated paint and lacquer as well as recovery of the water-treating agents so that waste disposal and waste water treatment costs can be minimized.